The present invention relates in general terms to alternators or starter alternators for motor vehicles, and in particular to the regulation of such alternators.
The regulation of the output voltage of an alternator is conventionally performed by comparing a signal taken at the output of the alternator with a reference voltage. In the simplest regulators, this reference voltage is fixed. In more sophisticated systems, the reference voltage may be made to vary according to the environment of the alternator (in terms of factors such as temperature, magnitude of the electrical load supplied at a given instant, and so on).
The most modern alternators offer the possibility of modifying the regulation reference voltage by communication of data with a remote intelligent device, such as a central control unit of the vehicle which is dedicated to the control of various components, such as the engine control apparatus or the remote control of various electrical loads, for instance.
One known solution for indicating to an alternator regulator circuit the reference voltage which it must adopt consists in making use of a pulse width modulated signal, also referred to in this specification as a PWM signal, in which the width of the pulses determines the value that the reference voltage must assume. Conventionally, the PWM signal received is applied to an integrator circuit of greater or less sophistication, which delivers an output signal consisting either of the reference voltage itself, or a voltage which enables the required correction to be made to a basic reference voltage. Such an approach is relevant in that it enables variations in the period of the received PWM signal to be cancelled out, so that only the cyclic ratio of this signal is taken into account.
One difficulty with this known conversion technique lies in the fact that, in order to perform the integration, it is necessary to have relatively high values of capacity (which are typically of the order of one or more tens of nanofarads), which prevents incorporation of the whole of the integrator circuit actually within an integrated circuit such as that on which the regulator is implemented. Thus, as a minimum, such a regulator makes it necessary to have one or more discrete capacitances connected to specific terminals of the integrated circuit.
The present invention aims to overcome the above limitations of the current state of the art, and to propose a signal for conversion of a received PWM signal which can be made entirely in integrated technology, while eliminating problems of variation of period in the PWM signal, and at the same time being capable in all cases of delivering with precision the voltage represented by the PWM signal.
According to a first aspect of the invention, an alternator for a motor vehicle, comprising a rotor and a stator and a regulator circuit for varying the excitation of the alternator by comparison of a signal representing the output voltage of the alternator with a variable reference voltage, and a conversion circuit provided to vary the reference voltage as a function of a reference control signal in the form of a pulse width modulated signal, is characterized in that the conversion circuit comprises, in combination:
an internal clock with a controllable variable period;
a differential circuit provided to establish a difference signal between the period of the reference control signal and the period of a signal from the internal clock;
a control circuit for the internal clock, adapted to control the internal clock in response to the difference signal in such a way as to equalize the period of the internal clock signal and the period of the control signal; and
a circuit for converting pulse width into voltage, comprising a counter which is paced by the controllable internal clock and which is adapted to perform a counting operation so long as the reference control signal is at a given logic level, and a digital/analogue converter which is adapted to convert a value of the count supplied by the counter into a voltage from which the reference voltage of the regulator is obtained.
According to the invention in a second aspect, an interface device for constituting the interface between a control apparatus which supplies a reference control signal in the form of a pulse width modulated signal, and a regulating device for a motor vehicle alternator, wherein the variations in width of the pulses in the signal are arranged to be converted into variations in a reference voltage of the regulating device, is characterized in that it comprises, in combination:
an internal clock with a controllable variable period;
a differential circuit adapted to establish a difference signal between the period of the reference control signal and the period of a signal from the internal clock;
a control circuit for the internal clock, adapted to control the internal clock in response to the difference signal in such a way as to equalize the period of the internal clock signal and the period of the control signal; and
a circuit for converting pulse width into voltage, comprising a counter which is paced by the controllable internal clock and which is adapted to perform a counting operation so long as the reference control signal is at a given logic level, and a digital/analogue converter which is adapted to convert a value of the count supplied by the said counter into a voltage from which the reference voltage of the regulator is obtained.
Some preferred, but not limiting, features of the alternator and of the interface device according to the invention, which may be adopted alone or in any technically feasible combination, are set forth below.
The difference circuit may comprise means for producing a symmetrical rectangular signal, the period of which is a whole number multiple of the period of the reference control signal.
The difference circuit may comprise a means for producing pulses representing the difference between the symmetrical rectangular signal and a signal produced from the internal clock.
The width of the difference pulses may be proportional to the difference between the period of the reference control signal and the period of the signal from the internal clock.
The difference circuit may include a means for producing a signal representing difference in direction, at least during the duration of the difference pulses.
The control circuit for the internal clock may comprise a bidirectional counter adapted to receive the difference pulses and the direction difference signal, together with a digital/analogue converter receiving the output from the bidirectional counter.
The internal clock may comprise a voltage controlled oscillator.
The digital/analogue converter of the conversion circuit may have a memorization input, while means may be provided for applying to the input a memorization signal during which the reference control signal is at a logic level other than the given logic level, each time that the counter has performed an acquisition of the value of the cyclic ratio of the reference control signal.
The conversion circuit/interface device may be made entirely in integrated technology.
The conversion circuit/interface device may be formed on a common semiconductor chip with the regulator circuit.
Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of a preferred embodiment of the invention, and of some modified versions, all of which are given by way of non-limiting example only and with reference to the accompanying drawings.